Move DefaultErrorHandlingMode to request options and add AllowErrorHandlingModeOverride

[tobias-tengler]

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[Copilot]

Pull request overview

This PR relocates null-propagation error handling defaults from schema/gateway options into request-level executor options, and introduces an explicit AllowErrorHandlingModeOverride switch to control whether per-request onError values are honored. It updates both Core and Fusion execution paths, plus related tests and migration guidance.

Changes:

• Move DefaultErrorHandlingMode from schema/Fusion core options into RequestExecutorOptions and FusionRequestOptions.
• Gate per-request onError overrides behind the new AllowErrorHandlingModeOverride setting in Core and Fusion execution.
• Update tests, snapshots, and migration docs to reflect the new configuration surface.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
website/src/docs/hotchocolate/v16/migrating/migrate-from-15-to-16.md	Updates migration guidance to configure the default mode via request options.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/SourceSchemaErrorTests.cs	Adjusts Fusion integration test setup to use request options for default error handling.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/FusionRequestOptions.cs	Adds Fusion request-level default error handling settings and clone support.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/FusionOptions.cs	Removes default error handling from Fusion schema-level options.
src/HotChocolate/Fusion/src/Fusion.Execution/Execution/Extensions/FusionRequestContextExtensions.cs	Switches Fusion runtime fallback logic to request options.
src/HotChocolate/Core/test/Execution.Tests/Errors/__snapshots__/NullErrorPropagationTests.PerRequestOverride_IsIgnored_When_AllowErrorHandlingModeOverride_IsDisabled.json	Adds expected Core snapshot for disabled per-request override behavior.
src/HotChocolate/Core/test/Execution.Tests/Errors/__snapshots__/NullErrorPropagationTests.DefaultErrorHandlingMode_AppliesFromRequestExecutorOptions.json	Adds expected Core snapshot for request-executor default mode behavior.
src/HotChocolate/Core/test/Execution.Tests/Errors/NullErrorPropagationTests.cs	Renames and extends Core tests for request-executor defaults and override gating.
src/HotChocolate/Core/src/Types/SchemaOptions.cs	Removes schema-level storage/copying of DefaultErrorHandlingMode.
src/HotChocolate/Core/src/Types/IReadOnlySchemaOptions.cs	Removes the schema-options API surface for default error handling mode.
src/HotChocolate/Core/src/Types/Execution/Processing/OperationContext.Pooling.cs	Reads null-propagation behavior from executor request options during operation initialization.
src/HotChocolate/Core/src/Types/Execution/Options/RequestExecutorOptions.cs	Adds request-executor-level default mode and override toggle.
src/HotChocolate/Core/src/Types/Execution/Options/IErrorHandlerOptionsAccessor.cs	Exposes the new error handling settings through the options accessor interface.

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